Desensitization of difluoroamino-substituted propellant compounds using dinitrogen tetraoxide

ABSTRACT

1. THE METHOD OF DESENSITIZING TO SHOCK A DIFLUOROAMINO-SUBSTITUTED COMPOUND OF THE FORMULA   (F-)2-N-C(-R)(-R&#39;&#39;)-C(-R&#34;)(-R&#34;&#39;&#39;)-N(-F)2   WHEREIN R, R&#39;&#39;, R&#34;, AND R&#34;&#39;&#39; ARE EACH EMEMBERS SELECTED FROM THE GROUP CONSISTING OF HYDROGEN AND BRANCHED AND STRAIGHT CHAIN LOWER ALKYL GROUPS OF UP TO FOUR CARBON ATOMS, THE TOTAL NUMBER OF CARBON ATOMS PRESENT IN R, R&#39;&#39;, R&#34;, R&#34;&#39;&#39; NOT TO EXCEED SIX; SAID METHOD COMPRISING ADMIXING THE SAID COMPOUND DINITROGEN TETRAXOXIDE IN THE MOLAR RATIO OF DIFLUOROAMINO-SBUSTITUTED COMPOUND TO DINITROGEN TETRAOXIDE OF 1.0:0.004 TO 1.0:0.3.

B. D. ALLAN ETAL DESENSITIZATION OF DIFLUOROAMINO-SUBSTITUTED PROPELLANTFeb. 9, 1971 COMPOUNDS USING DINITROGEN TETRAOXIDE Filed Sept. 10, 1962SENSITIVITY V. MOLAR RATIO a 0 o 0 8 7 6 5 zu wv. z w3 w MOLAR RATIOI,2-8|S(DIFLUOROAMINO)-2-METHYLPROPANEI DINITROGAN TETRAOXIDE nite 3 562035 nnsENsirizATioN on l)IFLUOROAMINO-SUB- STITUTED PROPELLANT COMPOUNDSUSING DINITROGEN TETRAOXIDE Barry D. Allan, Edgar F. Croomes, and WalterW. Wharton, Huntsville, Ala., assignors to the United States of Americaas represented by the Secretary of the Army Filed Sept. 10, 1962, Ser.No. 222,738 Int. Cl. C06b 19/06 US. Cl. 149-74 3 Claims The inventiondescribed herein may be manufactured and used by or for the Governmentfor governmental purposes without the payment of any royalty thereon.

This invention relates to the stabilization of rocket fuels employed inliquid monopropellant and liquid bipropellant systems. Moreparticularly, this invention is concerned with the stabilization ofdifiuoroamino-substituted compounds employed as fuels and/or oxidizersin rocket propulsion systems.

Recent developments in rocket fuels have been directed towards thepreparation of propellants which will afford higher specific impulsesthan those propellants utilized heretofore. Theoretical calculationsindicate that propellants based on acarbon-hydrogen-oxygen-nitrogenflourine system will exceed appreciablythe specific impulse of propellants based only on acarbon-hydrogenoxygen-nitrogen system. Therefore, a whole new class ofpropellants based on difiuoroamino-substituted compounds has evolved.

One problem immediately became apparent in work withdifluoroamino-substituted fuels. These fuels, because of the sensitivityof diliuoroamino groups, had a tendency to detonate at low-shooksensitivity values. This sensitivity requires the use of specialhandling techniques to prevent premature detonation of the fuel.

For application to rockets, especially military rockets, specialhandling requirements for a fuel are a distinct disadvantage and mayeven eliminate the fuel or rockets employing the fuel fromconsideration. To render these new fuels acceptable to military needsand thereby afford military rockets the advantages of the higherspecific impulse of these fuels required a means of reducing their shocksensitivity.

It has now been determined that these difiuoroaminosubstituted fuels maybe effectively desensitized without adversely affecting the propulsionperformance thereof by adding trace amounts of dinitrogen tetraoxide tothe fuel.

In View of the above, it is the object of the invention to set forth aneifective means of desensitizing difluoroamino-substituted compounds byadding to said compounds dinitrogen tetraoxide thereby preventingpremature detonation due to shock.

The manner in which this and other objects may be achieved will becomeapparent from the following discussion.

The graph illustrates the amount of desensitization achieved when atypical difluoroamino-substituted compound is treated with dinitrogentetraoxide.

The liquid difiuoroamino-substituted compounds desensitized thus farappear to be completely miscible with the dinitrogen tetraoxide,Therefore, all that is required to achieve the desensitization of thesecompounds to shock is a simple mixing of the two components. The molarratio of the difiuoroaminio-substituted compound to the dinitrogentetraoxide should be at least 1.020.004. There is little if anyadvantage in empolying a molar ratio in excess of one mole of dinitrogentetraoxide to each mole of difluoroamino-derivative.

For ordinary use as rocket propellant components, thedifiuoroamino-derivatives are rendered sufficiently insensitive to shockwhen a molar ratio of difiuoroaminoderivative to dinitrogen tetraoxideof 1.0:0.004 to 10:03 is employed. However, because the dinitrogentetraoxide can function as an oxidizer for thedifiuoroamino-derivatives, the use of more than trace amounts ofdinitrogen tetraoxide with the difluoroamino-derivatives is completelyacceptable.

In mixing the dinitrogen tetraoxide with the difluoroamino derivative tobe desensitized, the relatively low boiling point (B.P. 21.3 C.) ofdinitrogen tetraoxide should be considered. Working with temperaturesbelow this point prevents undue loss of the desensitizing materialduring mixing and storage. Nevertheless, since even trace amounts of thedesensitizer function successfully, low temperature requirements are notnecessary but merely advantageous.

The manner in which desensitization is achieved is not completelyunderstood. However, it is believed that the decomposition of thedifluoroamino derivatives, which accompany the shock and lead todetonation, is terminated by a reaction between the decompositionproduct of the difluoroamino derivative and the nitrogen dioxide whichis present in the dinitrogen tetraoxide. This reaction halts thedecomposition before detonation occurs. The nature of the reaction isunknown but apparently the available electron in the nitrogen dioxidereadily reacts with the decomposition product. Nitrogen dioxide is theproduct of the dissociation of dinitrogen tetraoxide in solution (seeMoeller, Inorganic Chemistry, published by John Wiley and Sons, Inc.,New York, 1952).

There is some evidence that the desensitization is achieved according tothe postulated method. If trace amounts of 1,1 diphenyl 2(2,4',6'trinitrophenyl)- hydrazine is added in trace amounts to the liquiddifiuoroamino-substituted compounds, desensitization is also achieved.Like the nitrogen dioxide, this compound also furnishes an availableelectron which apparently enters into some type of reaction with thedecomposition products resulting from shock. Thus, it would appear thatany substance which is capable of producing freeradicals would functionas desensitizers. However nitrogen tetraoxide is the preferreddesensitizer of the in vention.

It is to be understood that the above discussion is merely an attempt toexplain the desensitization process and is in no way to be interpretedas a limitation of the invention. Regardless of the mechanism by whichdesensitization is accomplished, the method of the invention fordesensitizing difiuoroamino substituted compounds achieves the desiredresult, the mechanism being immaterial to the successful employment ofthe method of desensitization.

Among the difluoroamino-substituted compounds which have beendesensitized by the dinitrogen tetraoxide are the tetrafluorohydrazineadducts of ethylenically unsaturated compounds such as 2-methyl-propene,4-methyl-pentenel, and methyl methacrylate. Generally speaking, theseadducts are prepared by reacting the unsaturated compound withtetrafiuorohydrazine at sub-atmospheric or super-atmosphereic pressure.The operable temperature range for the process is from 40 C. to C. Sinceone mole of tetrafluorohydrazine is required for each double bond in theunsaturated compound, at least one mole of tetrafiuorohydrazine isrequired for every mole of monoethylenically unsaturated reactant.However, an excess of up to five moles of the tetrafiuorohydrazine willincrease the yields and/or shorten the reaction time.

A preferred class of tetrafiuorohydrazine adducts suitable as propellantfuels which can be desensitized according to the process of theinvention are those of the formula o--o R l IF2 Nl 2 R'" wherein R, R,R", and R' are each members selected from the group consisting ofhydrogen and branched and straight chain lower alkyl groups of up tofour carbon atoms. The total carbon content of R, R, R, and R should notexceed six, however.

The following examples are illustrative of the preparation of thesedifluoroamino adducts.

EXAMPLE I 1,2-bis (difluoroamino) propane To a 500 cc. evacuated Pyrexbulb equipped with stopcock and condensing arm was added in vacuo 6.4 10mole crude tetrafluorohydrazine and 6.1 10 mole propylene. The bulb,equipped with a capillary monometer, was placed in an oil bath andmaintained at 100 C. to 110 C. for eight hours. The pressure decreasedfrom 518 mm. (106 C.) to 316 mm. (106 C.) during this period. The bulbwas then removed from the bath and the condensing arm immersed in a bathat -l96 C. A slight pressure of noncondensable gas was pumped from thebulb, the material remaining in the bulb being subsequently warmed toroom temperature and transferred into the vacuum system. Somenon-volatile material remained in the bulb. The material whichtransferred to the vacuum system was fractimated through traps. Thefraction collected at 96 C. was a colorless liquid with a reproduciblevapor pressure of 21 mm. at C. A molecular weight determination on analiquot of the fraction by the gas density method gave value of 146.9; CH N F requires 146.1. The fraction Weighed 0.45 gram which correspondsto a 60% yield based on tetrafluorohydrazine consumption.

Calculated for C H N F (percent): C, 24.66; H, 4.14; N, 19.18; F, 52.02.Found (percent): C, 24.72; H, 4.74; N, 18.05; F, 49.9.

EXAMPLE II 1,2-bis (difluoroamino) propane A 1.5 :1 molar ratio oftetrafluorohydrazine to propylene was heated at 110 C. in a 500 cc.stainless steel bomb for two and one-half hours. The pressure decreasedfrom 72 psi. (25 C.) to 21 psi. (25 C.) during this period. The productwas distilled under an atmosphere of nitrogen yielding almost 6 grams of1,2-bis (difluoroamino) propane having a boiling point of 8384 C. at apressure of 760 mm. This corresponds to approximately a 90% yield basedon the propylene employed.

EXAMPLE III 1,2-bis (difiuoro amino -2-methyl-prop ane Following theprocedure of Example I, a mixture of tetrafluorohydrazine andisobutylene, in a molar ratio of tetrafluorohydrazine to isobutylene of1.27:1, was heated at 110 C. for six hours. During the course of thereaction the pressure of the mixture dropped from 496.2 mm. (28.1" C.)to 95 mm. (252 C.). After heating, the reaction mass was cooled to roomtemperature. The desired product was again recovered by fractionatedyielding 3.6 g. (86% yield) of a liquid having a boiling point of99.5100 C. The product gave vapor pressure of 12.1 mm. at 00 C., 26.1mm. at 15.0 C., and 49.3 mm. at 27.8 C. Quantitative hydrolysis of 61.06mg. in 35.99 ml. of 0.1110 N sodium hydroxide-10 ml. ethanol solutionfor twenty-one hours gave an equivalent Weight of 40.14; calculatedequivalent weight 40.0.

Calculated for C.,H N F (percent): F, 47.5. Found (percent): F, 47.2.

By substituting methyl methacrylate, 4-methyl-pentene- 4 1, butene-l,and other ethylenically unsaturated compounds for the propylene andisobutylene of Examples I-III, the tetrafluorohydrazine adducts of thesecompounds can be synthesized.

The degree of desensitization achieved through the use of dinitrogentetraoxide was determined through the use of the Olin Mathieson dropweight tester. This test has been approved by the Joint-Army-Navy-AirForce Panel on Liquid Propellant Test Methods. Essentially, this testeris used to determine what shock will cause the non-desensitized materialto detonate fifty percent of the time. The shock .is achieved bydropping various weights measured distances to subject the materialbeing tested to the impact thus produced. Then the material isdesensitized and sub- 1 jected to the test again. The increase in theamount of weight and/or distance the weight is dropped to produce fiftypercent detonations when compared to the weights and distances for thenon-desensitized material provides a means of evaluating the degree ofdesensitization accomplished.

in kg.-cm. and is the product obtained by multiplying the weight whichis dropped, measured in kilograms, times the distance the weight isdropped, measured in centimeters.

The 'E value for the pure 1,2-bis (difluoroamino)-2- methyl-propane is6.9 kg.-cm. Since the graph does not clearly denote the amount ofdesensitization achieved when dinitrogen tetraoxide is employed in traceamounts,

some E values for those trace amounts are given hereinafter. When themolar ratio of 1,2-bis (difluoroamino)- 40 2-methyl-propane todinitrogen tetraoxide is 1.020.004, the

E value is 41 kg.-cm.; when 1.020.03, the E value is 46 kg.-cm.; when1.0:0.56, the E value is 103 kg.-cm.

The amount of desensitization achieved with other homologous members ofthe previously identified preferred class of tetrafluorohydrazineadducts is closely analogous to that shown in the graph for theisobutylene adduct.

Another distinct advantage of the desensitizing agent of the inventionis its compatibility with the difluoroaminosubstituted fuels.Compatibility studies show that the reactivity of dinitrogen tetraoxidetowards the difluoroamino derivatives is dependent upon theconcentration of dinitrogen tetraoxide and the temperature. When a molarratio of the tetrafiuorohydrazine adduct of isobutylene to dinitrogentetraoxide of 10:15 was subjected to prolonged heating at C., reactionor decomposition definitely occurred. This problem is also encounteredwith other difluoroamino derivatives if the concentration of dinitrogentetraoxide is relatively high (for example, onehalf mole to one andone-half moles of dinitrogen tetraoxide to difluoroamino derivative) andthe temperature is maintained at 70 C. or higher. However, prolongedheating of the tetrafiuorohydrazine adducts of the alkenes attemperatures of 50 C. to 60 C. does not give evidence of reaction ordecomposition even at higher levels of dinitrogen tetraoxideconcentration. Illustrative of this is the absence of any reaction ordecomposition when a molar ratio of the tetrafluorohydrazine adduct ofisobutylene to dinitrogen tetraoxide of 1.0215 is maintained at atemperature of 50 C. to 60 C. for several days. When only trace amountsof dinitrogen tetraoxide are present there is no problem ofdecomposition or reaction below 100 C.

The difluoroarnino derivatives desensitized according to the method ofthis invention can be employed in the bipropellant rocket motors.Suitable oxidizers for the fuels include red fuming nitric acid anddinitrogen tetraoxide. The fuel and oxidizer can be injected into thecombustion chamber and ignited in a conventional manner as, for example,by first igniting a butane and air mixture. However, the operation ofrocket engines is well within the skill of the art and no detaileddiscussion is necessary herein.

From the foregoing discussion it should be readily apparent to thoseskilled in the art that the shock desensitization method of theinvention is broadly applicable to desensitizing anydifiuoroamino-substituted compound, particularly liquid compounds.However, solid difluoroaminosubstituted compounds which are soluble inliquid dinitrogen tetraoxide or are soluble in a liquid which is also asolvent for dinitrogen tetraoxide can be desensitized according to themethod described herein.

Moreover, while the major use for difiuoroamino-substituted compounds atthe present time is as rocket fuels, this method of desensitization willfind further application when the uses for difluoroamino derivativesspread to other areas.

While the invention has been described by reference to specific detailsthereof, it is not intended that the invention be construed as limitedto these details except insofar as they appear in the appended claims.

We claim:

1. The method of desensitizing to shock a difluoroamino-substitutedcompound of the formula PJ/NB NF2\R' wherein R, R, R", and R are eachmembers selected from the group consisting of hydrogen and branched andstraight chain lower alkyl groups of up to four carbon 6 atoms, thetotal number of carbon atoms present in R, R, R, R' not to exceed six;said method comprising admixing with said compound dinitrogentetraxoxide in the molar ratio of difiuoroamino-substituted compound todinitrogen tetraoxide of 1.0:0.004 to 1.0203.

2. The method of desensitizing to shock1,2-bis(difluoroamino)-2-methylpropane by admixing therewith dinitrogentetraoxide in the molar ratio of 1,2-bis(difluoroamino)-2-methyl-propaneto dinitrogen tetraoxide of 1.010.004 to 1020.3.

3. The method of desensitizing to shock the tetrafiuorohydrazine adductof a mono-ethylenically unsaturated compound, said adduct being theaddition reaction product of one mole of tetrafiuorohydrazine with onemole of said compound and being characterized by the presence of thegroup" said method comprising admixing dinitrogen tetraoxide with saidadduct, the amount of adduct to dinitrogen tetraoxide being in a molarratio of 1.020.004 to 10103.

BENJAMIN R. PADGETT, Primary Examiner U.S.Cl.X.R.

1. THE METHOD OF DESENSITIZING TO SHOCK A DIFLUOROAMINO-SUBSTITUTEDCOMPOUND OF THE FORMULA